The mined crude oil tends to be increasingly heavy and, on the other hand, the demand for heavy oil tends to decrease. Therefore, in petroleum refining, it is desirable to crack any produced residual oil as effectively as possible to thereby raise the clean oil yield. Moreover, in accordance with the decrease of natural oil reserves, attention is being drawn to the effective utilization of superheavy crude oil such as oil sand or Orinoco tar.
For example, with respect to the direct utilization of vacuum residual oil, there can be mentioned the use of the vacuum residual oil as a heavy oil prepared by cutting back with gas oil or as road construction base materials. On the other hand, with respect to the method of upgrading the vacuum residual oil, there can be mentioned the method of producing light hydrocarbon such as fluid catalytic cracking, hydrogenation cracking or thermal cracking and the physical separating method in which deasphalted oil (oil from which asphaltene was removed) is extracted with the use of a light hydrocarbon, e.g., propane or butane as a solvent.
As compared with the gravity decrease through cracking, the upgrading through solvent deasphalting is advantageous in that the apparatus is relatively cheap and hydrogen is not used. However, the solvent deasphalting residuum is solid at ordinary temperature, so that the upgrading through solvent deasphalting has the disadvantage that the handling thereof for stocking or transportation is not easy. When the solvent deasphalting residuum is used as a liquid fuel, about 30 to 50% by weight of cracked gas oil is added to the solvent deasphalting residuum so that the viscosity thereof is reduced to the same level as that of heavy oil. However, this method has the drawback that the cracked gas oil obtained by a fluid catalytic cracking of deasphalted oil is used as a cutter stock with the result that the extraction ratio of solvent deasphalting is lowered. Therefore, the water slurry forming technique in which the solvent deasphalting residuum is ground and dispersed in water at a high concentration is drawing attention.
The conversion of coal as a solid fuel to a liquid fuel in the form of a water slurry (Coal Water Mixture; hereinafter also referred to as "CWM") has already been brought into practical use. However, with respect to the heavy carbonaceous residuum from petroleum such as the solvent deasphalting residuum, there are not only peculiar technical problems not experienced in the conversion of coal to a water slurry, for example, the problem that the softening point thereof is so low that the residuum is susceptible to temperature atmosphere to thereby cause the handling to be difficult but also inherent technical problems that cannot be coped with in the same manner as in the production of CWM regarding, for example, the grindability and dispersibility, realizable concentration and product stability in slurry formation. Therefore, research is being promoted toward the practical conversion of the heavy carbonaceous residuum from petroleum to a water slurry.
Generally, the most important technical requirements to be satisfied by the slurry fuel or by the process for producing the same are the capability of preserving the fuel solid component at a high concentration, low slurry viscosity, stability during the storage and transportation and reduction of cost incurred by the grinding energy, apparatus, dispersant,etc. It is desired that all of these requirements be collectively satisfied.
First, with respect to the concentration of fuel solid component among the slurry product characteristics, the below described Japanese Patent Laid-open Publication No. 62(1987)-225592 points out that the closest packing principle common to CWM is believed to be also applicable to the solvent deasphalting residuum-water slurry (Residue-Water Mixture; hereinafter also referred to as "RWM"). That is, when the target apparent viscosity at 20.degree. C. is 1000 centipoise (cP) or less, the practically possible maximum concentration is about 65 to 70% by weight or slightly over the same in terms of fuel solid component concentration. Furthermore, the fluidity and stability of the pumpable water slurry is susceptible to the variety, concentration, particle diameter distribution and dispersion state of fuel solid component particles as the principal component, the variety and amount of added dispersant, the variety and amount of stabilizer for sustaining the stability of the slurry, the mutual functional relationship of all the constituent elements including these, the atmosphere such as temperature, the production conditions, etc.
The preferred particle size distribution of solvent deasphalting residuum for obtaining a slurry with high fluidity while maintaining the fuel solid component at a high concentration is known to be in the form of approximately an inverted W character over the particle diameter range of about 1 to 1000 .mu.m as shown in FIG. 9. The reason is that particles with small diameters enter gaps among particles with large diameters so that the particles of solvent deasphalting residuum are brought into the closest packed state to thereby enhance the fluidity of the water slurry. On the other hand, when the particle diameters are uniformalized, gaps are formed among the particles, irrespective of the magnitude of the particle diameters, with the result that the closest packed state cannot be realized.
Moreover, although the particle size distribution can be shifted toward the small diameter side (left side of FIG. 9) while maintaining the above form of approximately inverted W character, obtaining such a particle size distribution is practically infeasible in view of the structure of the apparatus for agitating and grinding the solvent deasphalting residuum. For example, when it is intended to prolong the agitation period to thereby reduce the particle size, only the particles with large diameters have their sizes reduced while the particles with small diameters are no longer ground. As a result, the large diameter end of the particle size distribution graph of FIG. 9 is abruptly deviated toward the small diameter side (left side of FIG. 9) so that a sharp peak is realized to result in a degradation of the fluidity of the water slurry. Contrarily, when the particle size distribution is deviated toward the large diameter side (right side of FIG. 9), the amount of particles precipitated in the water slurry is increased because the particle diameters become large to thereby result in a degradation of the long-period stability of the water slurry.
The process for producing the solvent deasphalting residuum-water slurry (RWM) will now be studied. It was anticipated that the typical process employed in the production of coal-water slurry (CWM) would be applicable, as a practical economic process, to the production of RWM. Specifically, it was anticipated that, for example, the one-stage grinding process comprising performing a wet high-concentration fine grinding of coarsely ground raw material in the presence of a dispersant in water, followed by addition of a stabilizer and blending together, would be applicable to the production of RWM.
Therefore, the inventors have attempted to grind the solvent deasphalting residuum with the use of ball mill grinding apparatus having been used in the production of CWM. However, the obtained ground particles have the particle diameter range deviated toward the small diameter side, and the particle size distribution of broad particle diameter range as shown in FIG. 9 has not been obtained. The reason would be attributed to a significant difference in concentration, dispersion state and stability between the water slurry from solvent deasphalting residuum and the water slurry from coal, this difference resulting from a constituent component difference such that the oil content, bubble, heavy metal content and sulfur content of solvent deasphalting residuum are more than those of coal, or a difference therebetween in specific gravity, ground particle configuration and grinding characteristics, or a difference therebetween in slurry forming conditions.
The inventors have accordingly conducted extensive and intensive studies on the grinding of solvent deasphalting residuum with the use of the ball mill grinding apparatus. As a result, it has been found that the particle size distribution with the form of approximately an inverted W character over a broad particle diameter range as shown in FIG. 9 can be obtained by a two-stage grinding.
Two-stage grinding with the use of the ball mill grinding apparatus is unfavorable because the number of steps is increased to thereby increase the production cost with the result that the target of converting the residuum to fuel with minimized cost cannot be attained.
Another technique in which the two-stage grinding is performed is described in Japanese Patent Laid-open Publication No. 62(1987)-225592. In the process of this publication, use is made of a grinding apparatus comprising an oblate cylindrical grinding chamber and, fitted therein at slight spacings from its upper and lower surfaces and circumferential side wall, rotary blades or grinding blades being a combination of rotary blade and fixed blade. Feeds from a hopper are ground by means of this grinding apparatus, and obtained grinds are discharged through a discharge pipe. However, the inventors have empirically seized that the grinds with the desired particle size distribution as shown in FIG. 9 cannot be obtained by grinding the solvent deasphalting residuum by means of the above grinding apparatus. The reason is presumed to be that, in this process, the grinding of the solvent deasphalting residuum is accomplished with the utilization of not only shearing force but also large frictional force, so that a high temperature is realized by the large frictional force when the grinding is conducted by this process to thereby cause part of the solvent deasphalting residuum from vacuum residual oil, whose softening point is generally in the range of about 120 to 200.degree. C., to soften during the grinding.
The present invention has been made in these circumstances. The object of the present invention is to provide a process for easily producing a cheap highly stable petroleum residuum-water slurry at low cost, in which grinds with a desirable particle size distribution are obtained from a petroleum residuum such as solvent deasphalting residuum by a one-stage slurry forming step.